This patent relates to an operation of a non-volatile memory device and, more particularly, to a programming method of a non-volatile memory device, which can reduce the width of a threshold voltage distribution of an erase cell.
Flash memory, that is, non-volatile memory is generally classified into NAND flash memory and NOR flash memory. NOR flash memory has a structure in which memory cells are independently connected to bit lines and word lines and is therefore excellent in a random access time characteristic, whereas NAND flash memory has a structure in which a plurality of memory cells are connected in series, requiring only one contact per cell string, and therefore has an excellent characteristic in terms of the level of integration. Accordingly, the NAND structure is generally used in high-integrated flash memory.
A well-known type of NAND flash memory device includes a memory cell array, a row decoder, and a page buffer. The memory cell array is comprised of a plurality of word lines extending along rows, a plurality of bit lines extending along columns, and a plurality of cell strings corresponding to the bit lines.
A data storage state of a memory cell of the flash memory device is discriminated according to the threshold voltage Vt classified depending on the number of electrons stored in the floating gates. The number of data bits that can be stored in a memory cell depends upon the number of threshold voltage distributions that can be represented by the memory cell.
In general, a single level cell (SLC) includes only threshold voltage distributions representing two states, e.g., erased and programmed states, enabling data bits ‘1’ or ‘0’ to be discriminated.
If a memory cell can be made to have four threshold voltage distributions data can be discriminated like ‘11’, ‘10’, ‘01’, and ‘00’. Thus, one memory cell can store 2-bit data therein. This memory cell is called a multi-level cell (MLC).
For an MLC to store the 2-bit data therein it must have three threshold voltage distributions at voltage levels of 0 V and higher. Thus, the widths of the threshold voltage distributions need to be made narrow. To affect programming of the MLC, the program voltage applied during programming is increased within a small width.
However, threshold voltage distributions of a MLC become wide because of problems, such as interference, back pattern dependency, and source line resistance, as well as the program voltage problem.
FIG. 1 is a diagram showing threshold voltage distributions changed by environmental factors of memory cells.
FIG. 1 illustrates an ideal narrow width threshold voltage distribution 110 that, whereas a threshold voltage distribution 120 is influenced by back pattern dependency, a threshold voltage distribution 130 is influenced by source noise, and a threshold voltage distribution 140 is influenced by an interference phenomenon. Each of the voltage distributions 120, 130 and 140 has a wide width.
In particular, it can be seen that the threshold voltage distribution 140 affected by the interference phenomenon has the widest width. The interference phenomenon is generated by the influence of memory cells adjacent to a specific memory cell.
FIG. 2 is a diagram showing capacitance between peri cells, which generates an interference phenomenon between a memory cell and the peri cells. This drawing shows first to ninth memory cells C0 to C8 of the plurality of memory cells of a flash memory array. Capacitance coupling is generated between floating gates between the second to ninth memory cells C1 to C8 surrounding the first memory cell C0.
If, after the first memory cell C0 is programmed, the threshold voltage of any one of the neighboring second to ninth memory cells C1 to C8 is changed by a program voltage, the threshold voltage of the first memory cell C0 is changed due to capacitance coupling.
FIGS. 3A and 3B illustrate coupling capacitance due to an interference phenomenon between memory cells. This drawing shows the occurrence of coupling capacitance between floating gates FG. This coupling capacitance is a physical factor in terms of the structure of a memory cell. The coupling capacitance is proportional to an area A and also proportional to the dielectric constant of dielectric material therebetween, but is in inverse proportion to a distance t, as indicated by a capacitance equation of a general capacitor.
As the size of a flash memory device reduces, the distance between memory cells, e.g., “t”, decreases. Thus, the capacitance problem must be solved by looking to changes in the height of a floating gate, shielding of the dielectric material, reduction in the dielectric constant, and so on.
Further, available methods of preventing the interference phenomenon can include reducing the occurrence of interference coupling by relatively increasing the gate coupling ratio or changing a programming method from a random method to a sequential method. Alternatively, a method of minimizing a degree that the threshold voltage distribution of a memory cell is changed for a program can be used.
FIGS. 4A and 4B illustrate the shift of threshold voltages depending on assignment of data codes. Referring to FIG. 4A, data ‘11’, ‘10’, ‘00’, and ‘01’ are respectively set to memory cells having first to fourth threshold voltage distributions 411 to 414 in order of higher threshold voltage distributions. In this state, a least significant bit (LSB) program operation is performed such that some of the memory cells having the first threshold voltage distribution 411 are shifted and therefore included in the second threshold voltage distribution 412.
Next, through a most significant bit (MSB) program operation, some of the memory cells having the first threshold voltage distribution 411 are shifted and then included in the fourth threshold voltage distribution 414, or some of the memory cells having the second threshold voltage distribution 412 are shifted and then included in the third threshold voltage distribution 413.
As can be seen from FIG. 4B, a case where the shift amount of the threshold voltage is the greatest when programming the memory cells is the case where some of the memory cells having the first threshold voltage distribution 411 are shifted and then included in the fourth threshold voltage distribution 414. As the shift amount of the threshold voltage increases as described above, memory cells are influenced by more interference. Thus, a method of setting the program codes and performing a program as shown in FIG. 4B can be used. In this case, it can be seen that the shift amount of the threshold voltage by the program is reduced.
However, although this method is used, it is ineffectual as the distance between memory cells is narrowed with shrinking of the size of the memory device leading to an increase in the interference ratio.